DEVELOPMENT AND INTERCALIBRATION OF MONAZITE (U-TH)/ HE THERMOCHRONOLOGY: CATNIP SILL, CATALINA CORE COMPLEX

In recent years, monazite has become a target for (U-Th)/He thermochronology because of its petrogenetic significance. Moreover, high concentrations of both U and Th (up to 2 and 15 wt.% respectively) facilitate measurement of He isotopic ratios for single monazite crystals with (U-Th)/He ages as young as a few million years. Previous diffusion studies have demonstrated that monazite yields thermal history data within the range of 250–180°C, an important range for studying tectonic processes. The temperature range, however, further underscores the need to perform diffusions experiments on each crystal.

In this study, we present (U-Th)/He and ⁴⁰Ar/³⁹Ar thermal history data gathered for co-existing monazite, K-feldspar, muscovite and biotite. These data are modeled together to 1) assess the compatibility of the age and diffusion data from each phase, and 2) to arrive at a self-consistent thermal history. We focus on a deep-seated intrusive rock that was sampled from the Catalina metamorphic core complex near Tucson, AZ. The two-mica Catnip Sill of the Wilderness peraluminous intrusive suite crystallized in the lower crust in the Eocene and was exhumed in the middle Miocene.

We present new ⁴⁰Ar/³⁹Ar muscovite and biotite fusion ages, ⁴⁰Ar/³⁹Ar K-feldspar multi-diffusion domain results and (U-Th)/He monazite data. K-feldspar, muscovite and biotite results indicate rapid cooling from 450 to 150°C from 27 to 23 Ma. Initial experiments with monazite show that calculated closure temperatures for He diffusivity from this sample range from 226–208°C, depending upon composition and grain size. Monazite (U-Th)/He data from these grains yielded ages of ~27-21 Ma. These data demonstrate that integration of monazite (U-Th)/He with ⁴⁰Ar/³⁹Ar data from other minerals can constrain robust, self-consistent thermal histories for rocks exhumed through middle to upper crustal levels.